1. Field of the Invention
The present invention relates to an optical bend measurement apparatus.
2. Description of the Related Art
FIGS. 16 to 18 show a curvature measurement apparatus using an optical fiber, which is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 57-141604. FIG. 16 shows the state of the optical fiber before bending in the curvature measurement apparatus. FIG. 17 shows the state of the optical fiber during bending in the curvature measurement apparatus.
The optical fiber has a core portion 501 made of glass, transparent plastic, or the like. Part of the outer circumferential surface of the core portion 501 is covered by a light absorber 503, and the remaining part is covered by a cladding portion 502. Light impinging on the interface between the core portion 501 and the cladding portion 502 at an angle larger than the total reflection angle is totally reflected at the interface.
Light propagates through such optical fiber as follows. That is, in the state before bending shown in FIG. 16, a light ray 504 traveling parallel to the inner wall is transmitted. In light traveling not parallel to the inner wall, a light ray 505 impinging on the light absorber 503 is absorbed by the light absorber 503, and thus is not transmitted. Light not impinging on the light absorber 503 upon entering at a small angle like a light ray 506 is transmitted without being absorbed.
In the state during bending shown in FIG. 17, since light travels straight, all the light rays 504, 505, and 506 impinge the light absorber 503, are absorbed by the light absorber 503, and thus are not transmitted.
FIG. 18 shows a case in which such curvature measurement apparatus is applied to bend measurement of a rail of a railway. An optical fiber bundle 509 is arranged along a rail 508 of the railway, one end portion of the optical fiber bundle 509 is connected with a laser light source 510 and the other end portion is connected with a photoelectric conversion apparatus 511.
The optical fiber bundle 509 is formed from three optical fibers each including a light absorber 503 in correspondence with one of three measurement locations of the rail 508. The light absorbers 503 of the optical fibers are arranged at the measurement locations, respectively. The attenuation quantity of light transmitted through each of the three optical fibers is measured, thereby obtaining the curvature at a corresponding one of the three measurement locations.
One optical fiber may be used for measurement, instead of using the three optical fibers. In this case, the light absorber 503 is moved along the rail 508 to perform measurement three times, thereby obtaining a similar result. Alternatively, three absorbers 503 may be provided on an optical fiber at three locations. In this case, the attenuation quantity of light is obtained as the product of light attenuation quantities at the three locations.
As described above, the conventional curvature measurement apparatus measures the curvature of the rail or the degree of depression of the rail during train passage by measuring the attenuation quantity of light.
In the above-described conventional apparatus, a curvature obtained through a single optical fiber is a value at a location or the product of values at locations. To obtain individual values at locations, it is necessary to repeatedly lay a single optical fiber and perform measurement, or to lay optical fibers and perform measurement.
Such method is applicable if the volume presents no problem when laying optical fibers, for example, if the measurement target is a rail. However, such method cannot be adopted if it is difficult to lay optical fibers in a measurement target in terms of volume, or if it is difficult to change the laying conditions. Furthermore, the conventional apparatus cannot obtain curvatures at locations as individual values using a single optical fiber.